1. Field of the Invention
The present invention relates to a laser scanning apparatus and an image forming apparatus, and more specifically, to a laser scanning apparatus for scanning a surface to be scanned with beams and an image forming apparatus having the laser scanning apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus using a laser has been widely used for recording an electrophotographic image. In this case, the image forming apparatus includes a laser scanning apparatus and generally uses a method that forms latent image by a rotation of a drum while scanning laser beam to an axial direction of a photosensitive drum using a polygon scanner (for example, a polygon mirror). In the field of the electrophotograph, the image forming apparatus capable of realizing high density image for improving image quality and of outputting image at high speed for improving operability is needed.
As a method capable of achieving the high density and the high speed, a so-called multibeam that simultaneously scans a surface of the drum with a plurality of beams is proposed.
In Japanese Patent Application Laid-Open No. 2002-131662, for example, a laser scanning apparatus is disclosed, including a light source that has a plurality of light emitting devices two-dimensionally arranged; an optical sensor that is capable of detecting light beams deflected by a deflection unit to scan a surface to be scanned emitted from the light source at a particular position within a scanning range of the light beams; a generating unit that generates synchronous signals whose signal levels are varied according to an amount of light energy received in the optical sensor; and a controller that light beams each of the light emitting devices, within a period where the light beams emitted from the plurality of light emitting devices traverse a light-receiving surface of the optical sensor, the light emitting devices being previously selected as light emitting devices used for generating the synchronous signals and positions of the light emitting devices along a scanning direction of light spots formed on the surface to be scanned by the emitted light beams being almost equal to one another.
In Japanese Patent Application Laid-Open No. 2006-350167, a laser scanning apparatus is provided, comprising: a light source that emits a plurality of beams; a coupling optical system that couples the plurality of beams with a subsequent optical system; an optical deflection unit that deflects the plurality of beams to a main scanning direction; and a scanning optical system that images the plurality of beams deflected by the deflection unit on a surface to be scanned, the light source having a plurality of openings that limits a plurality of light emitting areas in surface emitting lasers where plurality of the light emitting areas are arranged in a form of a two-dimensional array, the plurality of openings having a one-to-one arrangement corresponding to each light emitting area, and if Dm is a width of the main scanning direction of the opening, Ds is a width in a sub-scanning direction of the opening, βm is magnification in the main scanning direction of the overall optical system, βs is magnification in the sub-scanning direction of the overall optical system, ωm is a light spot size in the scanning direction formed on the surface to be scanned, and ωs is a light spot size in the sub-scanning direction formed on the surface to be scanned, the scanning optical system satisfying conditions of Dm·|βm|<ωm and Ds·|βs|<ωs.
In recent years, the image forming apparatus is used in a simple printing as an on-demand printing system so that image quality of higher definition is demanded.
In the laser scanning apparatus that obtains synchronous signals of a scan start, etc. by receiving beams for synchronization formed of a plurality of beams in a light-receiving device, the scanning position of the beams for synchronization deviates from the sub-scanning direction due to a machining error or an assembling error of an optical device to reduce light intensity of the beams for synchronization received in the light-receiving device so that the predetermined synchronous signals are not obtained and the synchronous signals are unstable.